# PCR and Recombinant DNA Technology

> PCR and recombinant DNA for FMGE Biochemistry: cycle temperatures, Taq polymerase, restriction enzymes, vectors and CBNAAT in TB.

- Canonical URL: https://prepelephant.com/topics/fmge/biochemistry/pcr-recombinant-fmge
- Exam / course: FMGE · Subject: Biochemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "PCR and Recombinant DNA Technology", PrepElephant, https://prepelephant.com/topics/fmge/biochemistry/pcr-recombinant-fmge

## Direct answer

One DNA template molecule becomes more than a billion copies within two hours when the polymerase chain reaction cycles 30 to 40 times through denaturation at 94-95°C, primer annealing at 50-65°C and extension at 72°C by thermostable Taq DNA polymerase from Thermus aquaticus — every product becomes a template, so amplification is exponential. Recombinant DNA technology stitches DNA across species: type II restriction endonucleases cut palindromic sequences (EcoRI recognises GAATTC) leaving sticky ends, foreign DNA is ligated into a plasmid with origin and resistance marker, bacteria are transformed, and blue-white screening through lacZ picks recombinant clones. Human insulin (1982), hepatitis B vaccine and erythropoietin are the landmark products.

## What you must remember

- **PCR components:** template, a paired set of primers flanking the target (they define specificity), Taq polymerase, dNTPs, magnesium and buffer; 2 to the power n copies after n cycles.
- **Variants:** reverse-transcriptase PCR (RNA to cDNA first — viral loads and gene expression), real-time or quantitative PCR (fluorescence per cycle, the CT value), multiplex PCR and nested PCR for sensitivity.
- **Clinical deployments in India:** CBNAAT (cartridge-based nucleic acid amplification, GeneXpert) detects Mycobacterium tuberculosis and rifampicin resistance from sputum within two hours under the national programme; PCR underpins HIV and hepatitis C viral loads and SARS-CoV-2 diagnosis.
- **Restriction enzymes:** each recognises a specific palindrome and cuts to sticky or blunt ends — EcoRI (GAATTC), HindIII, BamHI; the fragments are separated by gel electrophoresis for DNA fingerprinting (microsatellite/VNTR-based).
- **Vector essentials:** origin of replication, selectable marker, multiple cloning site; bacteriophage lambda, cosmids and yeast artificial chromosomes carry progressively larger inserts; expression vectors add promoters and tags.
- **Blotting recap:** Southern for DNA, Northern for RNA, Western for protein; allele-specific oligonucleotide probes diagnose sickle cell and thalassaemia genotypes after PCR.
- **Applications ladder:** gene cloning, site-directed mutagenesis, transgenics, gene therapy vectors and DNA vaccines; forensic identity from minimal biological material.

## A worked case: two hours to rifampicin resistance

A 34-year-old with two months of cough and weight loss, previously treated once, produces scanty sputum — smear shows scanty bacilli. Rather than waiting weeks for culture and an indirect susceptibility test, the sample enters a CBNAAT cartridge: the machine lyses bacilli, amplifies the rpoB gene region by PCR in real time, and probes the rifampicin-resistance-determining region; MTB is detected and rifampicin resistance is flagged, reclassifying the patient as presumptive multidrug-resistant tuberculosis the same afternoon and routing him to a drug-resistance survey site. The molecular logic is sequence-specific hybridisation: wild-type probes fluoresce over the target; a mutation dropping a probe's signal marks resistance. This is PCR as public-health infrastructure — same-day infectious diagnosis and simultaneous resistance prediction — which is why it appears in FMGE stems on tuberculosis and on molecular diagnostics alike.

## Where students slip

The temperature triple is misremembered as one step: 94-95°C denatures, 50-65°C anneals (primer-dependent), 72°C extends — and the annealing temperature, set by primer melting temperature, is what actually tunes specificity. Second, Taq polymerase lacks 3'→5' proofreading, so high-fidelity polymerases (such as Pfu) are used when sequence accuracy matters; crediting Taq with proofreading is an error. Third, primers, not polymerase, define what is amplified — the polymerase merely extends whatever the primers bind. Fourth, in blue-white screening the white colony is the recombinant one: insertion disrupts lacZ, so no β-galactosidase cleaves the chromogenic substrate. Fifth, blot-name swaps (Northern and Southern) remain the single most frequent one-mark loss in this zone; anchor them by the inventor's surname (Southern = DNA) and the compass joke that followed.

## Frequently asked questions

### What are the three temperature steps of PCR?

Denaturation at 94-95°C, primer annealing at 50-65°C, and extension at 72°C, repeated 30-40 cycles for exponential amplification.

### Why is Taq DNA polymerase used?

It survives the denaturation temperature, being derived from the thermophilic bacterium Thermus aquaticus — though it lacks proofreading activity.

### What does CBNAAT detect in tuberculosis?

Mycobacterium tuberculosis and rifampicin resistance within about two hours by real-time PCR of the rpoB rifampicin-resistance-determining region.

### What are sticky ends in recombinant DNA work?

Single-stranded overhangs generated when restriction enzymes cut palindromic sequences off-centre, allowing complementary fragments from any source to anneal before ligation.

### Which blot detects each macromolecule?

Southern for DNA, Northern for RNA and Western for protein — with allele-specific probes after PCR enabling direct genotype diagnosis.